The ability to determine the RMS value of an unknown, e.g., AC signal with a high degree of accuracy is of critical importance in many environments. Presently, AC voltmeters provide accuracy to about 0.1% (1000 part per million). When higher accuracy is required, transfer standards are used. Transfer standards are used to measure the RMS value of an unknown AC signal by determining the difference between the RMS value of the unknown AC signal and a preset, accurately measured DC equivalent. While the use of transfer standards provides accuracy in the 100 parts per million (ppm) range, it has a number of disadvantages. First, the cost of the transfer standards test equipment required to achieve this accuracy is higher than desired. Secondly, and more importantly, the time required to make a transfer standards measurement having an accuracy in the 100 ppm range normally requires several (e.g., 5) minutes. As a result, the use of a transfer standards approach to determining the RMS value of a signal is both costly and time consuming. Thus, a need exists for an inexpensive measuring system for producing a signal that accurately represents the RMS value of an unknown AC signal.
While AC voltmeters and transfer standards have been utilized in the past to accurately determine the RMS value of an unknown signal, other, less accurate and substantially less expensive, devices have been developed for converting an unknown AC signal into a DC signal having a magnitude that is equal to the RMS value of the AC signal. One form of such systems applies the AC signal to be converted to a first heating element such as a thermal resistor. The heat produced is thermally coupled to a suitable heat sensor, such as a transistor, which is connected in a differential circuit with a similar heat sensor. The differential output is utilized to control the DC power applied to a second heating element thermally coupled to the second heat sensor. At balance, the DC feedback voltage applied to the second heating element is equal to the RMS value of the unknown AC signal applied to the first heating element. Devices of this type having a conversion accuracy of 0.5% have been produced. Such a device is disclosed in U.S. patent application Ser. No. 842,972, filed Oct. 17, 1977, by Roy W. Chapel, Jr. and I. Macit Gurol and entitled "Thermally Isolated Monolithic Semiconductor Die," now U.S. Pat. No. 4,257,061. While an accuracy of 0.5% (5000 ppm) is inadequate in many environments, these RMS converters have the advantage that they can be relatively inexpensively produced.
U.S. Pat. No. 4,274,143, entitled "Recirculatng RMS AC Conversion Method and Apparatus" describes a method and apparatus for providing highly accurate RMS conversion using a low cost thermal RMS converter. In this method and apparatus the signal whose RMS value is to be accurately determined is first converted into DC form by a relatively inaccurate thermal RMS converter. The result is a first converter signal (Y.sub.1), which is stored for recirculation in a suitable storage device, such as a sample and hold circuit. The first converter signal is also doubled (2Y.sub.1) and stored. Thereafter, the first converter signal stored in the storage device is recirculated to the converter to create a second converter signal (Y.sub.2). Then, the second converter signal is subtracted from the doubled first converter signal (2Y.sub.1 -Y.sub.2) to produce a highly accurate RMS output signal.
While the method and apparatus described in U.S. Pat. No. 4,274,143 is a substantial step forward in the art of accurately determining the RMS value of an unknown signal, it has one disadvantage. Specifically, the time required to accurately determine the RMS value of an unknown signal is greater than desired, particularly when a continuous determination is needed or desired. In this regard, in one actual embodiment of the invention described in U.S. Pat. No. 4,274,143 the time required to provide an accurate determination of the RMS value of an unknown signal is approximately six (6) seconds. While this time period is substantially less than the time required to make an accurate measurement using the prior art transfer standards technique [approximately five (5) minutes], this time period [e.g., six (6) seconds] is still longer than desirable when a continuous, accurate RMS conversion is required.
Therefore, it is an object of this invention to provide a new and improved high speed, highly accurate RMS conversion method and apparatus.
It is another object of this invention to provide a high speed, highly accurate RMS converter.
It is a further object of this invention to provide a highly accurate RMS conversion method and apparatus that rapidly and continuously converts an AC input signal into a DC signal having a magnitude proportioned to the RMS value of the AC signal.
It is yet another object of this invention to provide a highly accurate, high speed RMS converter system that uses a relatively inexpensive thermal RMS converter.